1. Field of the Invention
The present invention relates to cable handling systems, particularly systems for deploying and retrieving electrical and fiber optic cables. Most particularly, the invention relates to marine seismic cable deployment and retrieval systems for use in conjunction with a marine vessel.
2. Brief Description of Relevant Art
In many fields of endeavor, there is an on-going requirement to place packages of sensing equipment of various types across the earth's surface and on the seafloor. Such equipment is commonly intended to be used at one location for a period of time and then transported to a different location for further use. However, precisely deploying and later retrieving such equipment without damaging the equipment can be difficult. Operations in water, especially oceans, bays, and surf zones, can be especially problematic. The equipment commonly sinks into muddy and sandy sea beds and tends to suffer stress damage when removed.
Seismic cables can be especially difficult to handle because they are typically made of multiple components such as electrical conductors, fiber optics, and stress supporting members all bundled together and covered with a protective jacketing material. Handling or pulling the cable causes these components to slip and move with respect to one another. Tension applied to the outer jacket pulls the jacketing material which then pulls on the inner components of the cable. This distribution of stresses applies differing stress values and elongation amounts to the different components of the cable. Even cables where the stress members are embedded into the outer jacket have such a stress distribution, although to a lesser degree. Propagation of stress through a cable's components changes and deteriorates the components and consequently reduces the cable's useful life.
In water, the platform or vessel used to deploy and retrieve the cables often contributes due to the action of the water. Pulling cable up from a sea bottom and through sea bottom material is stressful to equipment in the cable, but simply pulling the cable through water is also stressful. Typically the cable will be curved in the water, extending downwardly from a platform and curving to a horizontal position along the sea bottom. The curve's length and shape will depend on the rate of retrieval, the depth of the water, the amount of cable sunk into the sea bottom, and the value of the applied pulling tension. The curve of the cable inevitably causes portions of the cable to be pulled sideways through the water, creating vortexes in the water, cable strumming, and drag on the cable, and adding further to the stresses on the cable. Such pulling tensions can exceed the strength of the cable, causing it to break. Similarly, tensions caused by pulling of the cable due to heaving of the vessel on ocean waves and swells can exceed the strength of the cable, causing it to suffer elongation damage and even break. The cable strength is commonly only a tiny fraction of the applied forces that potentially may be applied against the cable.
A need exists for systems and methods for deploying and recovering cables that reduce the destructive forces against such cables, particularly when the cables are distributed along a sea bed or in water.